The present invention relates to a method and apparatus for determining a vacuum degree within a flexible vacuum package containing a material in a vacuum state.
The evacuated heat insulation panel having a flexible covering plastic film bag, hereinafter referred to as PIP, is used in, for example, a heat insulation panel in the wall of a refrigerator and is produced as follows. An amount of a powder such as calcium silicate is charged into a metal deposited film laminate bag, which is then placed within a vacuum packer with its inlet opened. The vacuum packer is thereafter evacuated to a predetermined pressure at which the inlet of the bag is heat sealed. After the heat sealing, the PIP thus produced is subjected to a heat insulation test. The heat insulation test is carried out by means of a heat flow meter or a thermometer. This test is disadvantageous in that it takes a rather long period of time. This is due to the following reasons. Firstly, it takes time for the heat flow to become in a steady state due to a small thermal conductivity. Secondly, this test requires many data to be measured such as weight and size. Lastly, it is laborious to set a heat flow meter sensor and a cold terminal thereof.
As is well known, the degree of vacuum within the PIP is inevitably deteriorated with the lapse of time and thus the life of the PIP depends on this deterioration in vacuum degree. In practice, the amount of buildup or deterioration of vacuum is determined by measuring the degree of vacuum within the PIP twice and thereby the life thereof can be estimated. It is hard to predict the life of the product only by the widely-used heat insulation test using a heat flow meter.
It is also known that below a certain degree of vacuum the thermal conductivity of the PIP is constant. The determination of the vacuum degree of the PIP hence makes it possible to estimate both the heat-insulative property and the life of the PIP but measurement of these properties according to the prior art method is laborious and time consuming. Although the degree of vacuum of the PIP is determined by applying the conventional vacuum degree determination device to the PIP, this device considerably increases the equipment cost and may cause a leakage of the PIP. When the vacuum degree determination device is not available, the only method for determining degree of vacuum of the PIP is to break package of the PIP for the test.